This invention relates generally to frequency locking systems for television signal reception and demodulation, and more particularly the invention relates to circuitry which is immune to ghost signals which interrupt a transmitted television signal.
Echo signals or ghost signals have been one of the major problems in modern television transmission. When a transmitted signal is received from the air, ghosts can be caused by reflections from mountains, buildings, and the like. In a television signal received through cable, the ghost can be due to discontinuities of the connectors. Although the ghosts are generated in a radio frequency spectrum, the process in which the ghost is created can be accurately modeled as a linear distortion in a baseband signal. Therefore, cancelling the ghost can be accomplished by passing the baseband signal through a linear filter which is an inverse of the linear model of the ghosting process.
In a television equipped with a ghost cancellation system, one needs to run a digital or analog filter with a system clock whose frequency is locked to the video signal. Some ghosts can cause conventional line locked or burst locked systems to fail to acquire steady and clean lock. Usually, the sync separator can be confused by some ghosts, making the horizontal sync or burst gate unusable.
In conventional frequency lock system clock generators, the reference used is either a line locked (usually horizontal sync from a sync separator) or a color burst locked using the color burst and a burst gate signal derived from the sync separator. However, as noted above severe ghosts can cause the sync separator to fail thus producing jittery or missing reference horizontal sync or badly or missing burst gate timing. Further, the color burst may have colored ghosts from a previous line which cause contamination.